Generating power requires that gas turbines consume large quantities of air. Gas turbines are strongly dependant on the ambient air conditions for their performance. Ambient air conditions such as temperature, pressure and water content impact the gas turbine's compressor's capability to compress the air and thereby affects its performance. In other words, gas turbine power is a function of the total mass flow available for compression, in combination with fuel and expansion to drive a turbine section. Mass flow is directly proportional to the engine power output. Gas turbines are constant volume machines (i.e., they operate according to fixed geometries), and thus, air density is one parameter that plays an important role in a gas turbine's ability to generate power. Air temperature and air density are in direct correlation to one another. As air temperature increases the density of air decreases, thereby resulting in a decrease of the overall potential for mass flow. As mass flow decreases, the output of the gas turbine also decreases. Other key parameters that have a strong impact on the performance of the gas turbine include the pressure ratio and the compression efficiency.
Mass flow can be managed by manipulating the water vapor content at the air intake of the gas turbine. Thus, air can be saturated with water vapor to return the overall mass flow to the maximum level of the turbine's design. Saturation can result from simply saturating the air surrounding the gas turbine. Alternatively, a more aggressive approach to increasing overall mass flow is injecting water into the turbine's compressor or combustor to oversaturate the air. Oversaturation allows the heat of fusion to further pressurize the working fluid and increase the turbine's output to a level above saturated air output levels.
However, proper saturation of air can be problematic because of the range of temperatures encountered by the gas turbine throughout a given time period (i.e., temperature changes over a 24 hour period or over an annual time period). As a result of these temperature variances, the water requirement for saturation will vary accordingly. For a given weather and engine load situation, a corresponding amount of water is necessary to reach saturation or oversaturation. Thus, humidity detection and pumping equipment are used in providing the proper amount of water for the appropriate level of saturation or oversaturation. Using too much water results in “overspray,” where the air can not absorb/hold the excess water. The excess water can harm operations by corroding and/or flooding the gas turbine's air duct. On the contrary, too little water will not saturate the air and the full mass flow increase effect will not be accomplished.
Another issue is the build up of fouling or foreign particles in the turbine, particularly in the compressor, which can affect the gas turbine's efficiency and therefore its power output. Machines, such as gas turbines, consume large quantities of air. Air contains foreign particles in the form of aerosols and small particles, which typically enter the compressor and adhere to components in the compressor's gas path. Compressor fouling changes the properties of the boundary layer air stream of the gas path components because the deposits increase the component surface roughness. As air flows over the component, the increase of surface roughness results in a thickening of the boundary layer air stream. The thickening of the boundary layer air stream negatively effects the compressor's aerodynamics. At the blade's trailing edge, the air stream forms a wake. The wake is a vortex type of turbulence that has a negative impact on the air flow. The thicker the boundary layer the stronger the wake turbulence. The wake turbulence together with the thicker boundary layer has the consequence of reducing mass flow through the engine. The thick boundary layer and the strong wake turbulence result in a reduced compression pressure gain, which in turn, results in the engine operating at a reduced pressure ratio. A reduced pressure ratio results in a lower efficiency of the engine. Further, fouling of the compressor reduces the compressor isentropic and polytropic efficiency. Reduced compressor efficiency means that the compressor requires more power for compressing the same amount of air. As a result, the power required for driving the compressor increases and results in less surplus power being available to drive the load.
Washing the gas turbine counteracts the fouling and can be conducted either with the engine being shut down or during its operation. In the former instance the engine shaft can be cranked using its starter motor while wash water is injected into the compressor. Fouling is released by the act of the chemicals and the mechanical movement during cranking. The water and the released fouling material are transported to the exhaust end of the engine by the air flow. This procedure is called “crank” washing or “off-line” washing. An alternative to off-line washing is “on-line” washing where the engine is washed while in operation. “On-line” or “fired” washing occurs as the engine is firing fuel. The washing water is injected into the compressor while the rotor is spinning at high speed. Due to high rotor speeds and short retention time for the water this wash is not as efficient as the crank wash, but allows washing during operation.
Typically, attempts to augment gas turbine power have utilized extensive instrumentation throughout the entirety of the turbine for measuring temperatures, displacement, pressures and loading levels of the machine. However, power augmentation that relies on such extensive instrumentation is problematic due to its expense, complication of use, and increase in the potential and likelihood for the occurrence of operational errors resulting from instrumentation inconsistencies or failure. Eliminating such reliance on complex and extensive instrumentation to augment power of a gas turbine is desired.
Thus, there exists a need in the industry for a method and apparatus for augmenting the power output of gas turbine engines including, but not limited to, a stationary gas turbine engine, over a wide range of operating conditions where the system is controlled using a computational fluid dynamic transfer model.